You enlighten me, despite it works a bit improperly with my code.

It seems that  the AssistantPaneDialog OnSize(...) cx, cy not match it's size.So after I correct the cx,cy according it's size, the problem disappears.

void AssistantPaneDialog::OnSize(UINT nType, int cx, int cy)
{
 RECT rcWindow;
 GetWindowRect(&rcWindow);
 CDialog::OnSize(nType, rcWindow.right-rcWindow.left,  rcWindow.bottom-rcWindow.top);
 RepositionBars(0, 0xffff, AFX_IDW_PANE_FIRST, 0, 0,0);
}

Just call RepositionBars in the end of OnInitDialog.

Any suggestions...

 

I put two dialog derived panes(MainPaneDialog,AssistantPaneDialog) in the main frame(Hide Client) ,and nest two dialog  derived panes(ChildPaneDialogOne,ChildPaneDialogTwo) in the AssistantPaneDialog(Hide Client).

 

Pic Startup:

Pic AfterResize

 

When the program startup, the child panes' sizes are not properly.(Pic Strartup). After a frame resizing, the child panes' sizes are properly(Pic AfterResize).

 

It's properly that parent(AssistantPaneDialog) dialog template size is smaller than it's pane size. When the child panes show, they use the parent dialog template size.And no further WM_SIZE message is sent to the parent pane to correct it's client size and Reposition Bars.So some parent pane's area is left.